An example of such a slider is described in the patent EP 204 186 B1 held by YKK, and another example is illustrated in Swiss patent application 01697/03 filed by the present applicant.
In both of the examples, the slider is locked in its position by means of a pointed member which penetrates between the teeth of the fastener, and in order to move the slider it is first necessary to disengage this member, which can also be called a “pawl”, from the teeth.
To ensure that this locking operation can be performed by pulling as stated on either one of the two tabs connected to the slider, the terminal rings of the tabs are connected to two levers, located inside the hollow body of the slider on opposite sides of the plane of the fastener, and interconnected by contact or by means of suitable return members. One of these levers is connected integrally to the aforesaid pawl, and the levers are designed and positioned in such a way that a pull on either one of the two tabs will directly or indirectly cause the movement of the pawl in such a way as to disengage it from the teeth of the fastener.
The manufacture of the levers described above and of the corresponding actuating systems entails production and assembly costs which, given the intrinsically relatively low value of a slider with two tabs, have a significant effect on the final cost.
Moreover, since an increase in the number of parts of an assembly also increases the potential causes of malfunction, the reliability of sliders made as described above can some times be inadequate, and in order to achieve correct operation it is necessary to use more costly methods and materials, which in turn have an additional effect on the price.